Method for producing cellulose esters



Patented May 23, 1939 2,159,643 PATENT OFFICE METHOD FOR PRODUCING CELLULOSE ESTERS Richard Weingand and Ernst Acker, Bomlitz, near Walsrode, Germany, assignors to Sylvania Industrial Corporation, Fredericksburg, Va., a corporation of Virginia.

No Drawing. Application November 10, 1937, Se-

rial No. 173,862. In Sweden May 4, 1935 8 Claims.

The invention relates in general to cellulose derivatives and, in particular, to a process for the preparation of a cellulosic'raw material adapted to be used in the preparation of cellulose derivatives andi to correlated improvements designed to enhance the properties and usefulness of the products so produced. it In making cellulose esters it is customary to usecellulose in the form of thickpaper boards l0 with a Weight of about 500 gbper square meter "rand with a specific gravity of about 0.6. When employing such cellulose boards for the este'rification, there are great difliculties due to the fact that the cellulose swells only to a small degree. "For instance, when esterifying cellulose boards by means of the known nitrating mixture consisting of sulphuric acid, nitric acid and water,

alsuperficial nitration of cellulose board takes place shortly after their introduction into the esterifying liquid. The nitrocellulose layer which is formed in this way at the surface inhibits the penetration of the mixed acid into the ,inner parts of the cellulose board and the removal :of the water liberated during the nitration. i In viewl thereof, it is hardly possible, even with a longer duration of nitration to obtain a final product which is uniformly nitrated throughout {the mass.

In order to avoid these drawbacks it hasalready been proposed to form the cellulose, by means of a paper machine, into very thin paper :sheets having a weight of about 20 to 30 g. per square meter. Furthermore, these paper sheets are creped in order to prevent the several nitrat'ed acid to penetrate between the different sheets. ;The.formation of cellulose into such thin creped sheets is expensive and even with such sheets a relatively large quantity of mixed acid is required for the nitration in comparison with the quantity "of acid necessary for nitrating an equivalent quantity of cotton linters. Furthermore, the

creped paper occupies in the nitrationreceptacle arelatively large volume due to its crepedcondition and, therefore, the capacity of an existent nitration plant becomes considerably reduced in comparison with the treatment of cotton linters. Therefore, it is a general object of the invention to overcome the above mentioned disadvantages of the prior art and to produce cellulose derivatives having improved properties from a 1 cellulosic raw material prepared in a simple and economical manner.

sheets from adhering together and to enable the,

a cellulosic raw material of such physical and chemical character that the properties of the cellulose derivative so produced are improved and the cost diminished.

It is a specific object of the invention toprovide a process of making nitrocellulose from a cellulosic raw material of the above mentioned characteristics.

According to the invention, cellulose derivatives of improved properties are prepared using as raw material a cellulose of high alpha cellulose content prepared by treating wood fibres first with a relatively dilute alkali at an elevated temperature and pressure to disintegrate the cellulose fibres and separate the non-cellulose substances therefrom, and thereafter refining the disintegrated fibres by treatment with a relatively concentrated alkali at atmospheric temperature and pressure to remove as by dissolving the noncellulose constituents without substantially mercerizing the cellulose, and purifying the product in a knownmanner. The purified cellulose fibres are felted into a sheet on a suitable paper machine without exercise of substantial pressure either during sheeting or during drying toproduce a substantially undegraded cellulose of above 93% alpha cellulose in the form of a thin sheet having a loose porous structure, a specific gravity of form not above 0.4 and an absorptivity for water of not substantially above 200 seconds.

The purified alpha cellulose in theform of a thin porous sheet of the above mentioned characteristics may be converted to cellulose derivatives, such as cellulose ethers and esters, in a known manner. In particular, a nitrocellulose suitable for gun cotton may be made by nitration of the paper sheet with a suitable acid, the product being stabilized and converted into explosive materials in a suitable manner.

The invention accordingly comprises a process having the several steps and the relation of one or more of such steps with respect to each of the others thereof and a purified cellulose sheet possessing the features, properties and the relation of constituents which are exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

, As raw material for use in preparing cellulose derivatives according to the invention, there may be employed any suitable cellulosic material of vegetable origin, in particular, wood pulp, such as spruce, pine and the like. The wood fibres are first disintegrated and separated by treating the Wood pulp with a dilute aqueoussolution of a strong alkali at an elevated temperature and elevated pressure. For example, wood pulp may be treated with an aqueous solution containing from about, per liter, 7.5% to 9.0% of sodium hydroxide and 1.5% to 2.0% of sodium sulfide. Preferably, the amount of alkali per weight of dry wood pulp is from about 18% to 20%, not including the kill ail.

- .ing the sheet.

amount ,of sodium sulfide asblack liquor which may be ,a dded-=thereto. The disintegratingtreatment is carried out at an elevated temperature, for example, at about 170 C. and at an elevated pressure, for example, at .8 atmospheres, .the treatment taking place in a suitable pressure chamber. After several hours treatment there is obtained a product which comprises chiefly ligno cellulose, the treatment having removed the incrustations of the wood, such forex-ample, as lignin, hemi-celluloses, sugar, resins, fats and other non-cellulosic constituents 50f the pulp. During this alkali treatmentathe fibresareseparated one from another, but they are not substantially gelatinized nor is the cellulose substantially degraded.

a'Il-he disintegrated fibres are subjected ,to an alkali refining treatment a designed to :remove ;the

non-cellulose substances from the fibres by dissolution. il hissecond aflkali treatmentis, char- :acterized by. employing a relatively concentrated aqueous solution containing from-,;9% to 20% of ,a,-s trong alkali. -,The ;ratio :of solid alkali to 1 dry cellulose: in this -treatment is about .7 :to1. The treatmentis carried out at atmospheric pressums and: at 'a'temperature such that the --cellu lose zfibres are not substantially :mercerized :or

.selatinized. Using .an aqueous solution of =l,0% sodium ihydroxidesa temperature of from 16 C.

tog-2010. is suitable. During this alkali refining treatment, -the1non-cellulose substances, :such vas, ":lignin, gpentosans, resin and fats :are ;.removed. The product may be washed -w-ith water and treate with suitable tmineral acid to {neutralize ithe residual alkali. After -centrifuging there ;is obtained an undegraded cellulose pulp icompris- .ing from 193% :to :95% ialpha cellulose.

.Ihis ,pu-rified alpha cellulose .pulp ;may;be 5511bjected to a :bleaching treatment, if desired. iEor :example, the alkali-purified cellulose may (be -blea ;hed with :an :aqueous solution of sodium hypoehloriteacontaining about .20 g.- of free chlomine ,per liter, the, temperature (of the bleaching solution :bein s'abou 2135 C. t 40 Th :bleaohed product is :theniwell washed to remove th ibleaehinslase t :FIO :p l pare the wood pulp for esterification, it is -preferable1to formgit into -aithin paper ,she'et rQfjlQQSBDOIOUS structure. For sheeting the-pulp, there may be :.employed -.a :long sieve :paper ima- .chineior a paper {machine of :the cylinder :sieve type. ipuringzthe sheetingthespaper is-carried through the machine without :subjeoting ill :to any,substantialpressureso as 'to :avoid 'oompact- 'I he wet sheet is :then :dried :IOVGI' a suitable drier which-does :not rsubject the sheet ltosubstantial pressure. 'By avoiding substantial pressure during -both --sheet ing and "drying -:there ,is iproduceda ,paper; sheet iwhich is icharacterized @by .having :a ,loose .porous structure, a specific gravity .{Qf .not ,above =-.3 V to -0.,4 rand an absorptivlty .for ,esterifieation acids ;of from 40 to :200 .seconds. It -is preferable --to .sheet I the ,material in such ,avmanner that .the dried :sheet has a thickness-of about 1.5 mm, although .the inven- ,tion.is,not1 to,be limited ;to sheets of-such thick- ,ness.

flihe absorption .time .is a -measurement of the time required for Water to travel shy capillary action between ;-two spaced rpoints on a paper strip, one {end :of WhiOh vis immersed in ;,water. 33o determine "the ,absorption itime, ,a cellulose striphavin sawwidth,of 20 mm.,.a length of 100 mm; and athickness of 1.5 mm. is :provided with elihI'QC imarzks positioned -at1distances of 10, mm,,

by washing the-nitrocellulose. maybe-stabilized'by'treatment with hot water. The major quantity of water is eliminated by .15;-mm,,-;a nd:65mm., respectively, from the lower iedge. "This strip .is dipped vertically into water up to the lower mark and the time required for athe water to rise from the middle to the upper .markismeasured.

The sheet of interfelted cellulose fibres of the present invention is especially adapted for conversion "to variouscellulose derivatives, such as cellulose ethers, for example, methyl cellulose,

ethyl cellulose, and cellulose esters, for example, cellulose nitrate, cellulose acetate, and the like,

in a-,;suitable manner.

For .example, the porous, highly absorptive purified cellulose sheets may be cut into small pieceshaving the area of about the palm of the the weight of the drycellulose. :Aftera suitable period of treatment the :main quantity of the ni-tratingaeidiseliminated bymeans of a hydroextractor and the acid stilladheringis removed The nitrocellulose means of ,aeentrifugal separator to such a degree ;that1the nitrocellulose contains about 35 per cent water. By varying the conditions of nitrationandthe ratio and concentration of the acids, there ,may be produced nitrocellulose adapted for use :in plastics, collodions, lacquers,

explosives, -etc.

f'lFor example, :totproduce smokeless powder, a

g,un eo tton :nitrocellulose obtained by means of this invention and having about 13.3 per cent nitrogen, will be mixed witha quantity of colglodion fibres necessaryefor-the gelatination and vmade of a :nitrocellulose containing about 12.3 :per :cent :nitrogen.

These collodionfibres are produced in a manner analogous to the manu- Then, the mixture 51S introduced into orifice. For instance, .for the production oil smokeless powdenthere areemployed orifices of rectangular cross section, whereby narrow fiat .bandsare obtained, and these bands are transformed, after partial removing of the solvents, .into thesmall :leaves as customary.

It ,is known that hemicelluloses are decomposed to a large extent by esterifying acids.

Therefore, the removal of the hemi-cellulose as effected by the present invention prevents the contamination of the esterifying acids, so that a higher yield of thedesired cellulose derivatives is obtained ,with a smaller quantity of acid. Moreover, when the present alkali purification treatments are prolonged, there is obtained a product having-an alpha cellulose content above 93% with which the yield of nitrocellulose is greater, and the consumption of the acid less than Withqth sulphite cellulose of the prior art.

Further, the purer alpha cellulose gives clearer solutions of the cellulose esters :produced there from.

hand. After a short preliminary drying, these pieces are ,treated atagtemperature of 15 ,C. to ,C for '30 minutes with a nitrating mixture composed of 23 per cent nitric acid, 67% sulphuric acid and ;10 ,;per cent water, the quantity 20 of ,mixed acids being about, forty or fifty times Since the alkali-purification treatment of the present invention is carried out without substantial mercerization, gelatinization or degradation of the alpha cellulose, the cellulose esters produced from such purified cellulose are equal to or better than those prepared directly from cotton linters. In particular, the less degraded cellulose of the invention produces a nitrocellulose of higher specific gravity and gives products, such as films, etc., of greater strength than those made from sulphite cellulose.

Byavoiding pressure during sheeting and drying of the purified cellulose pulp, the absorptivity of the sheets produced by the invention is greatly increased. The loose porous structure of the present sheet avoids the formation of superficial layers of cellulose ester on the sheet so that penetration of the esterifying acids is rapid and uniform throughout the sheet. In addition, the porous structure of the present sheet decreases the quantity of acid which remains adhering to the cellulose after squeezing and extraction of the acid, in consequence of which less acid is wasted in washing.

A further advantage of the loose porous structure of the present sheet is the fact that the absorption of the acid is very rapid, so that there is no need to crepe the sheet of the present invention as is necessary with those produced with sulphite cellulose of the prior art. By using the uncreped porous sheet of the invention the capacity of an esterification apparatus is fifty per cent greater than when creped sheets are employed.

temperature and pressure to disintegrate the wood fibres and to separate non-cellulose components therefrom, thereafter treating said disintegrated fibres'with a relatively concentrated .alkali containing from about 9 to 20% alkali at atmospheric temperature and pressure to remove the non-cellulose components from the fibres, forming said fibres into a sheet without subjecting said sheet to substantial pressure dur- 1 ing sheeting and drying.

2. A process which comprises treating cellulose wood fibres with relatively dilute alkali containing from about 7 to 9% alkali at an elevated temperature and pressure to disintegrate the wood fibres and to separate non-cellulose components therefrom, thereafter treating said disintegrated fibres with a relatively concentrated alkali containing from about 9 to 20% alkali at atmospheric temperature and pressure to remove the non-cellulose components from the fibres, forming said fibres into a sheet without subjecting said sheet to substantial pressure during sheeting and drying to form a product having an absorptivity for water of not substantially more than 200 seconds.

i 3. Aprocess which comprisestreating cellulose wood fibres with relatively dilute alkali containing from about 7 to 9% alkali at an ele vated temperature and pressure to disintegrate the wood fibres and to separate non-cellulose components therefrom, thereafter treating said disintegrated fibres with a relatively concentrated alkali containing from about 9 to 20% alkali at atmospheric temperature and pressure to remove the non-cellulose components from the fibres, forming said fibres into a sheet without subjecting said sheet to substantial pressure during sheeting and drying and converting the said sheet into cellulose derivatives.

4. A process which comprises treating cellulose wood fibres with relatively dilute alkali containing from about 7 to 9% alkali at an elevated temperature and pressure to disintegrate the wood fibres and to separate non-cellulose components therefrom, thereafter treating said disintegrated fibre with a relatively concentrated alkali containing from about 9 to- 20% alkali at atmospheric temperature and pressure to remove the non-cellulose components from the fibres, felting said fibres into a sheet without subjecting it to substantial pressure during sheeting and drying to produce a sheet having an absorptivity of not less than 200 seconds and converting said sheet into cellulose nitrate.

5. A process for making cellulose esters comprising esterifying a sheet of inter-felted substantially undegraded cellulose fibres substantially free of non-cellulosic constituents having a thickness of about 1.5 mm., a loose porous structure, a specific gravity of from 0.3 to 0.4 and an absorptivity for water of from 40 seconds to 200 seconds and produced in accordance with claim 1.

6. A process for making cellulose nitrate comprising nitrating a sheet of inter-felted substantially undegraded cellulose fibres substantially free of non-cellulosic constituents and having a thickness of about 1.5 mm., a loose porous structure, a specific gravity of from 0.3 to 0.4 and an absorptivity for water of from 40 second to 200 seconds and produced in accordance with the process of claim 1.

'7. -A sheet of inter-felted substantially undegraded cellulose fibres substantially free of noncellulosic constituents and having a thickness of about 1.5 mm., a loose porous structure, a specific gravity of from 0.3 to 0.4 and an absorptivity for water of from 40 seconds to 200 seconds produced in accordance with the process of claim 1.

8. The process for producing a sheet of interfelted cellulosic fibres adapted for conversion into cellulose derivatives, comprising treating cellulose wood fibres with an alkali solution having a concentration of from 7.5% to 9% by weight at elevated temperature and pressure, the amount of alkali being sufficient to give a ratio of solid alkali t0 cellulose of 1:5, to disintegrate the wood fibres and to separate non-cellulose components therefrom, thereafter treating said disintegrated fibres with an alkali solution having a concentration of from about 9% to 20% by weight at atmospheric pressure and temperature, the ratio of solid alkali to cellulose being from 0.7 to 1, to remove the non-cellulose components from the fibre and forming said treated fibres into a sheet without subjecting the fibres to substantial pressure during sheeting and drying.

RICHARD WEINGAND. ERNST ACKER. 

